1. Field of the Invention
The present invention relates to metal disintegration apparatus, and more particularly to a metal disintegration apparatus which can perform the desired cutting task on a workpiece at a remote and relatively obstructed location.
2. Description of the Prior Art
The present invention relates to apparatus for cutting unmachinable metals and alloys and metal heat treated to an unmachinable condition and is particularly concerned with the difficult task of slotting plates in tube heat exchangers for steam generators used in conjunction with nuclear reactors.
In tube type heat exchangers designed with a preheat section, a first fluid flows through the tubes of the heat exchanger while a second fluid enters through a preheater section consisting of baffles on the cold leg of the tube bundle such that the heat exchange occurs more effectively between the two fluids. Occasionally, due to flow induced tube vibrations, some tubes may experience rapid wear at the baffle plate junctures such that if a leak occurs therein the fluids are allowed to mingle. When such a situation occurs, plugging the damaged tubes and continuing with plant operation is not acceptable as loose parts can be generated. Therefore, a long term corrective action must be implemented in order to prevent inevitable operational limitations which are likely to have serious economic reprecussions.
Although there are several methods known in the art for repairing heat exchanger tubes affected by vibrational problems, many of these methods are not applicable to repair of heat exchanger tubes where the tube is not easily accessible. For instance, if drills are utilized, a thrust of force must be applied against the tube plate. This is a problem in view of the fact that the plates are often located at inaccessible positions within the steam generators. The use of flame operations such as in welding or cutting torches is not allowed in a nuclear environment because it would create air-borne contamination by radioactive particles.
Because of these difficulties, special methods have been developed for repairing heat exchanger tubes in nuclear steam generators. Typically, the repair method involves internal modification of the pre-heat section via the main feed water nozzle. Since most of the modifications to be done are at a location where access is very tight and the area is radioactive, exposure time for repair work is limited. An additional problem consists of the physical size and material combination of the actual internal structure which has to be removed or machined prior to installation of a new flow distribution device.
The present invention teaches an apparatus for machining rectangular slots in the plates enclosing the upper part of the water chambers of steam generators adjacent to the feed water inlet thereof, thus providing a flow bypass from the preheat section. As a result, feed water velocity is diminished in the preheat section and the flow is distributed in an orderly pattern. The present invention accomplishes this by providing an advanced electro-machining technique for removal of the slot material.
Generally, electro-metal disintegration machining has been used in machine shops wherein electrodes for a metal disintegration machine are formed in a desired shape to create the desired opening. Devices of this character are shown in U.S. Pat. Nos. 3,518,196 to Bonga, 3,806,691 to Roach, 3,833,785 to Roach, 4,203,018 to Schnieder, 4,215,261 to Brifford, 4,230,926 to Gaumond, and 4,243,836.
Additionally, U.S. Pat. No. 4,259,562 to Cammann teaches a portable disintegrator which has a small degree of reach so that metal disintegration can be accomplished at a location spaced from the vibrator thereof. In two embodiments an extended shaft is provided including a support, the entire shaft and support being vibrated by a vibrator. In another embodiment a rigid conduit is provided between the electrode of the metal disintegrator and the vibrator thereof, the motion of the vibrator being transferred to the electrode by a flexible push-pull cable. As for the embodiment wherein the entire shaft and support vibrates, this is extremely inconvienent and largely impractical for use at remote locations. As for the embodiment which includes a rigid conduit in which a flexible cable is reciprocated, because of the configuration of the conduit and cable and the driving thereof by a solenoid type vibrator, only relatively small displacements of electrodes relative to the vibrator can be effected.
Further, the device suffers from the disadvantages of a conventional push-pull cable in that it works well in tension but not particularly well in compression. In a metal disintegration machine when a vibratory motion and an advancing motion is transferred to an electrode over a remote distance, such as five feet or larger, the cable employed needs to work well both in compression and tension for movement of the electrode.
The present invention overcomes the shortcomings of the prior art by providing a metal disintegration apparatus which in one embodiment provides an electrode supported by a movable arm and which is driven by an actuator cable means such that once the arm is articulated into position only the cable, and thus the electrode, is moved. Another embodiment of the present invention provides a rigid conduit between the vibration means and the electrode means thereof, the vibration means driving the electrode means by virture of a special moving element assembly capable of working over distances much greater than a conventional push-pull cable.